London - Novak Djokovic prepared for Saturday's last-four clash with Kevin Anderson at the ATP Finals in ominous fashion by outlasting Marin Cilic 7-6 (9/7), 6-2 as Alexander Zverev geared up to face Roger Federer.
The Serbian world number one started Friday's evening match at London's O2 Arena with the luxury of knowing he had already qualified as the winner of the Guga Kuerten Group but looked sharp and focused from the first point.
Earlier, Zverev saw off John Isner 7-6 (7/5), 6-3 to reach the semi-finals of the season finale for the first time, meaning the dream Djokovic-Federer final remains a possibility.
Djokovic, dressed all in black, won a remarkable 31 points in a row on his serve from the third game of the first set until the fourth game of the second against a determined Cilic, who was unable to progress even with a victory.
"Obviously I knew before the match that I had already qualified, and I knew who I'm playing, so it wasn't easy to approach this match with 100 percent willingness to win it," said Djokovic.
"But this is a unique format and both of us wanted to win from the first point. It was a close first set which probably decided the match.
"(Anderson) is playing some great tennis, it's definitely his best season and he's continued doing that throughout this tournament. He's playing well, serving well, very aggressive, so I know what to expect."
In a high-quality contest, the 14-time Grand Slam champion stayed calm to save a set point in the first-set tie-break before taking it 9-7 on his second set point.
In the second set, Djokovic immediately put Cilic's serve under severe pressure.
The Croatian former US Open champion dug deep to stay in the contest in the early stages but cracked in the fifth game, giving himself a mountain to climb and also succumbed in his next service game, leaving Djokovic to serve out for the win.
Djokovic, who has not dropped a single set in his three round-robin matches in London, has enjoyed a remarkable renaissance in the second half of the season after elbow surgery and indifferent form earlier in the year.
He has lost just two matches since the start of July, winning Wimbledon and the US Open along the way and emerging once more as the man to beat as he attempts to pull level with Federer on six ATP Finals titles.
Zverev was aware that a win by any scoreline against American giant Isner would take him through to the semi-finals.
The turning point came at the end of the 12th game, when the 21-year-old German saved a set point on his own serve with an ace, going on to force a tie-break, which he won.
In the second set a single break of serve in the eighth game proved decisive and Zverev served out for the win.
"It's obviously great getting to the semi-finals," he said.
"But the tournament isn't over. I am in the semi-finals. There's only good opponents left. There's only the best in the world.
"I'm playing Roger tomorrow, which is going to be a very difficult but hopefully very nice match. We'll see how far I can go."
Third seed Zverev, coached by Ivan Lendl, is the youngest semi-finalist at the ATP Finals since Juan Martin del Potro in 2009.
Second seed Federer, chasing his 100th title, beat Anderson in straight sets to top the Lleyton Hewitt Group on Thursday but both players progressed, meaning early exits for Kei Nishikori and Dominic Thiem.
Results on the sixth day of the ATP Finals at London's O2 Arena on Friday (x denotes seeding):
Guga Kuerten Group
Alexander Zverev (GER x3) bt John Isner (USA x8) 7-6 (7/5), 6-3
Novak Djokovic (SRB x1) bt Marin Cilic (CRO x5) 7-6 (9/7), 6-2
Lukasz Kubot/Marcelo Melo (POL/BRA x3) bt Oliver Marach/Mate Pavic (AUT/CRO x1) 7-6 (7/4), 6-4
Pierre-Hugues Herbert/Nicolas Mahut (FRA/FRA x8) bt Mike Bryan/Jack Sock (USA/USA x5) 6-2, 6-2